Conventionally, there is known a plant growing method in which artificial light is irradiated to a plant to modulate its growth rate, bloom time, and the like. It is reported, for instance, that growth of a short-day plant such as chrysanthemum and a day-neutral plant is promoted by being irradiated with far-red light after sunset (see JP2006-67948A).
In the growth promoting method of the short-day and day-neutral plants, the plants are irradiated with monochromatic far-red light. The light irradiated to the plants, however, not necessarily monochromatic, but a plurality of lights with different wavelengths may be used.
As one example of the plant growing method by utilizing two kinds of lights with different wavelengths, it is reported that at least one of red light and far-red light is irradiated to a solanaceous plant (tomato) for 1 to 3 hours after sunset in order to obtain high sugar tomato (see JP2007-282544A).
As another example, a short-day-treatment method of accelerating a bloom time of a short-day plant, is disclosed (see JP2009-136155A). In this method, a short-day plant is irradiated with red light and far-red light simultaneously at both and/or either of near the beginning and the end of a day-light phase in a photoperiod of the short-day plant.
In the method disclosed in JP2006-67948A, however, the far-red light irradiated to a plant has only a weak growth promotion effect.
In the method described in JP2007-282544A, at least either of the red light and the far-red light irradiated to a tomato does not necessarily accelerate the growth of the tomato, although it increases the sugar content of the tomato. This method is limited to a tomato, but cannot necessarily be applied to other plants.
In the system disclosed in JP2009-136155A, the red light and the far-red light irradiated simultaneously to a short-day plant do not necessarily promote the growth of the short-day plant, although they accelerate the bloom time of the short-day plant.